Blender 2.81 render not using all GPUs
I had a similar problem a while back with cycles. I used a script for rendering on remote ubuntu systems with titans xps. Try running the following script IF you too have Titan xps, otherwise try to find the right blender-name for your GPU  first. This may or may not work, I thought you may want to give it a shot. 

import os, bpy

os.environ['CUDA_VISIBLE_DEVICES'] = '0,1'

bpy.context.scene.render.engine = 'CYCLES'
bpy.data.scenes["Scene"].cycles.device = 'GPU'
bpy.data.scenes["Scene"].cycles.feature_set = 'SUPPORTED'
bpy.context.preferences.addons['cycles'].preferences.compute_device_type = 'CUDA'

cprefs = bpy.context.preferences.addons['cycles'].preferences

# Enable all CPU and GPU devices
for device in cprefs.devices:
    if device.name == 'TITAN Xp':
        device.use = True
    else:
        device.use = False

# Tests
print('Compute_device_type: ', cprefs.compute_device_type)
for device in cprefs.devices:
    if device.use == True:
        print('USED: ', device.name)
    else:
        print('NOT_USED: ', device.name)
print('Will be used: ' + bpy.data.scenes["Scene"].cycles.device)
print('Feature set: ' + bpy.data.scenes["Scene"].cycles.feature_set)
print('Render engine: ' + bpy.context.scene.render.engine)